1. Field of the Invention
This invention relates broadly to arterial prosthesis. More particularly, this invention relates to vascular stents, and even more particularly to helical stents.
2. State of the Art
Transluminal prostheses are widely used in the medical arts for implantation in blood vessels, biliary ducts, or other similar organs of the living body. These prostheses are commonly known as stents and are used to maintain, open, or dilate tubular structures.
Stents are either balloon expandable or self-expanding. Balloon expandable stents are typically made from a solid tube of stainless steel. Thereafter, a series of cuts are made in the wall of the stent. The stent has a first smaller diameter which permits the stent to be delivered through the human vasculature by being crimped onto a balloon catheter. The stent also has a second, expanded diameter, upon the application, by the balloon catheter, from the interior of the tubular shaped member of a radially, outwardly directed force.
Self-expanding stents act like springs and recover to their expanded or implanted configuration after being compressed. As such, the stent is inserted into a blood vessel in a compressed state and then released at a site to deploy into an expanded state. One type of self-expanding stent is composed of a plurality of individually rigid but flexible and elastic thread elements defining a radially self-expanding helix. This type of stent is known in the art as a “braided stent”. Placement of such stents in a body vessel can be achieved by a device which comprises an outer catheter for holding the stent at its distal end, and an inner piston which pushes the stent forward once it is in position. However, braided stents have many disadvantages. They typically do not have the necessary radial strength to effectively hold open a diseased vessel. In addition, the plurality of wires or fibers used to make such stents could become dangerous if separated from the body of the stent, where it could pierce through the vessel.
Therefore, recently, self-expanding stents cut from a tube of superelastic metal have been manufactured. These stents are crush recoverable and have relatively high radial strength. Referring to prior art FIG. 1, WPO Patent Application WO 01/89421-A2 (with inventors Cottone and Becker, and referred to herein as “Cottone”) describes a self-expanding vascular stent 10 constructed with a helical structure 12 of hoops in the central portion of the stent, a cylindrical hoop 14 of hoops at each end of the stent, and a transition zone 16 joining each cylindrical ends 14 to the central helical portion 12.
The cylindrical-to-helical transition zone 16 is created by splitting a second set of hoops from a cylindrical “turn” so that a loose end results to connect directly to the helical portion. More particularly, Cottone shows a set of transition hoops beginning adjacent to the cylindrical portion, starting at 20 with very short hoops, and the length of the hoops increases circumferentially so that after one circumferential turn around the stent the hoop length at 22 is approximately two times the length of the very short hoop at the beginning of the transition hoops. Cottone shows the end 24 of the shortest hoop joining the middle of the straight leg 26 (the “strut”) of the longest hoop at a junction point 28. From the end of that longest straight leg a new set of hoops (beginning at 30) continues to form the helical central portion 12 of the stent 10. Thus, by joining the beginning 24 of the transition hoops (the “start” of the transition) to the strut 26 of the longest hoop (the “end” of the transition), a “free end” 32 is created that forms the beginning of the helical set of hoops. While this solves the need of creating a free end, it causes a problem because the strut 26 to which the end 24 is joined can not bend sharply at the junction point 28. As a result, there is insufficient flexibility in the short hoop 20 that begins the “start” of the transition. Indeed, the joining of the beginning of the transition section to the middle of the end hoop (the “junction point”) creates an overly-rigid portion of the transition zone 16. This rigidity is caused by the inability of the strut 26 of the long hoop 22 to move in the direction of the short attached “start” hoop 20.
The construction shown in Cottone causes the helical hoops 12 to be “out of phase” with the short hoops at the beginning of the transition portion 16. This is because from the junction point 28, the helical hoops begin with a “forward” strut 32, and the transition hoops begin with a “backward” strut 34. As such, connecting bridges 36 are in different orientations, preventing the stent from easy expansion and collapse.
In addition, referring to prior art FIG. 2, the transition zone 16 defined by Cottone is in the form of a generally triangular section in which each successive strut around the circumference from the start of the transition to the end of the transition is longer than the previous strut. If each strut is to contribute equally to the overall radial compressive stiffness and strength of the stent, the struts must be adjusted in width (or other changes made, such as adjustment of the width of the half-loops which connect adjacent struts). That is, a longer strut must be stiffer (by thickening either the width or thickness) in relation to its length so that it will “open” or expand with the same force as a shorter strut. In a triangular transition zone as described by Cottone, each individual strut must be designed such that its width is in approximate proportion to the cube root of the length.
U.S. Pat. No. 6,190,406 to Duerig et al. teaches that the width along the length of a strut is preferably variable and in proportion to the cube root of the distance from point along the strut to the end of the strut. Using the same analysis, it is clear that for a strut of constant width, that width should be in proportion to the cube root of the length of the strut if it is desired to have an even expansion of all the struts of the stent. Duerig does not teach struts of different lengths, but rather teaches how to make tapered struts that minimize the peak strains in a bending situation. Cottone teaches struts of different lengths in the transition zone, but does not address the problems caused by these struts having widely different stiffness.
Moreover, adjusting the width of transition struts to create the proper stiffness for their length causes design compromises because there is not necessarily enough space about the stent to make the width of a long strut at the desired dimension without taking space away from the shorter struts. Doing so would cause the struts to be unequally distributed around the circumference.